In order to get it to work properly, you need to make it degenerative (not enough gain to oscillate) on the fundamental and 3rd overtone around 69 MHz (only odd overtones work) but with enough feedback on the 5th overtone that it oscillates. When you can hear a signal near 115 MHz but NOT near 23 or 69 MHz, then your oscillator is probably working correctly.

Thanks a million for the tip, you explain that very clearly. I'm going to check those areas tomorrow night, before I do anything else.

-- A strong signal at 23 MHz (technically, 22.980 MHz). So strong at this frequency that it contained a rapid, jackhammer-like 'pulse'.-- A fair-strong signal at 45.960 MHz, we can just call this 46 MHz.-- A fair-strong signal at 68.935 MHz, considered 69 MHz for the simple purpose.

Here's a kicker: A really strong signal at 160.855 MHz, actually its stronger than the 115 MHz (114.980 MHz) I get. I'd have to assume it's a result of the 115 MHz (oscillation?) plus a 45 MHz I.F. in my scanner. But I'm just guessing here.

The 22.980 MHz signal was copied on my Tecsun PL-600 portable SW radio. In addition there was an identical signal at 22.065 MHz, using the same crystal, just not quite as strong. I'll assume it's the 22.980 MHz minus 2 multiples of that radio's 455 kHz I.F. Again, that's just a guess.

Ok, so are you suggesting that the crystal isn't good enough, or that the circuit isn't, because it doesn't suppress the 69 MHz oscillation? Just so you know, the crystal I'm using (marked "115") I pulled from an old 115 MHz VHF converter. I hope to hear back from you before I proceed expanding the circuit.

Yes, I think your crystal is oscillating on the fundamental frequency of 23 MHz. You'llstill hear the other harmonics, including the one at 115 MHz.

The frequency is probably off slightly because the crystal is operating in a fundamentalmode rather than an overtone.

For proper operation you need to have it oscillating on the 5th overtone at 115 MHz: you should NOT hear a signal at 23 or 69 MHz in that condition. That's not to say thatthe crystal is bad: overtone oscillators are not always trivial to adjust. I think you justneed to work on the oscillator a bit to get it to work on the proper overtone.

And, yes, I think the 22.065 MHz response is an image in the receiver: I've been fooledby that before when I was trying to adjust one of my rigs to the 10m band, and endedup with the VFO off by 910 kHz!

Thanks. Well, I could try that Butler 5th overtone circuit, but I'm reluctant to. Its more complex than I desire right now. Because, if it doesn't work for some reason, then I'll have to back-track through 20 parts, as opposed to, say, five. If you follow me. Like last night, for example, when I was playing with that simple test circuit, it didn't work/oscillate at first, so I went for the obvious suspect: the capacitor across C-E. I changed the value and then it worked.

I think I've seen simple overtone circuits where a tank is put at the collector, resonant to the overtone that's wanted. Perhaps I'll try something like that next.

Simplicity is not always determined by parts count in a circuit: often what is moreimportant is the reliability with which the circuit will operate without needing adjustments.

With tube circuits there was always an effort to squeeze maximum performance outof as few stages as possible, because each extra stage not only added the cost of thetube, but also the current draw for the filament and plate. But with transistors thereis little need for that - adding an extra 2 transistors really doesn't cost much.

We particularly see this with transistor biasing: initial circuits used just a high valueresistor in the base circuit, but it needed to be adjusted for each specific transistordue to the range of characteristics. Many modern circuits use 3 resistors: one in theemitter and two as a voltage divider on the base. That might seem like morecomplexity than necessary, but it allows the transistor current to be set fairly accuratelyover a wide range of devices without requiring any adjustment.

The same is true of overtone oscillators: I have a lot of different circuits to choosefrom. One problem with the "simple" ones is that they often depend on parasiticcapacitance or other stray coupling for proper feedback: if you use a different typeof transistor it might not have the same characteristics. I've discovered over timethat my circuits work more consistently when I used explicit components rather thanrelying on circuit strays, as that allows me to adjust them if needed. And again, it makes the circuit more likely to work.

One thing I like about the Butler is the ability to short out the crystal and adjustthe tuned circuit until you get a signal on about the desired frequency: that makesit easier to adjust than just fiddling with bits here and there to see what happens.

Actually, from looking at the original oscillator circuit again it might be working as a harmonic oscillator rather than an overtone oscillator.

A harmonic oscillator is designed to oscillate on the fundamental, with oneor more tuned circuits that extract just the desired harmonic from the originalsignal. That would mean that it really is oscillating on 23 MHz, but the finaltuned circuit is tuned to 115 MHz to emphasize that frequency to be senton the the mixer. Harmonic oscillators are usually easier to get going (thoughthe tuning of the output network has to be set properly to get the rightoutput) but the output is generally lower and all the other harmonics fromthe fundamental frequency will also show up in the output to some degree(which can lead to extraneous responses.)

The output stage is tuned by the 35pf trimmer capacitor and L4. If you've reducedthe capacitor to get the circuit to oscillate, then the tuned circuit probably is notresonant at 115 MHz, and you wont' get the proper mixer injection. It may be thatif the circuit is tuned properly you would actually get overtone operation.

The test oscillator circuit is clearly a fundamental oscillator - it contains no tunedcircuits. You need some sort of tuning in an overtone oscillator because the gainis generally higher at the fundamental frequency than at the desired overtone, and some sort of frequency selectivity is required to restrict oscillation only tothe overtone. With a harmonic oscillator you need a way of extracting just thedesired harmonic. So either approach will require a tuned circuit of some sort.

A 2N2222 probably isn't the best transistor in this application: it has a typical fT of300MHz, and a good rule of thumb is to use a device up to 1/10 of the fT, thoughthey can be pressed into service at higher frequencies. I've found some samplesof 2N2222A that will oscillate as high as 150 MHz, but a proper VHF transistorsuch as 2N5179, 2N3563, or MPS-H10 would be more likely to give good performance.Look for something with an fT of at least 600 MHz.

Common FETs such as the MPF-102 or 2N3819 will also work, and any of theoscillator circuits (including the Butler) can be modified to use FETs instead ofbipolar transistors with a savings of a bias resistor or two.

A 2N2222 probably isn't the best transistor in this application: it has a typical fT of 300MHz, and a good rule of thumb is to use a device up to 1/10 of the fT, though they can be pressed into service at higher frequencies. I've found some samples of 2N2222A that will oscillate as high as 150 MHz, but a proper VHF transistor such as 2N5179, 2N3563, or MPS-H10 would be more likely to give good performance. Look for something with an fT of at least 600 MHz.

Yeah, I know. It was either the 2N2222 or a 2N4401, but the latter has a lower fT so I chose the former. I have some J310's, which I know can operate in the VHF range, but so far I've only had success with BJT's, so that's what I used in the test circuit/oscillator. Tonight, however, I'm going to try this FET overtone oscillator, and see what it does for me. Plus its simple. Anyone who knows me, and works with me, knows that I like simple:(see Fig. 7-5)

Nothing. Zilch. Words can't describe how much I hate FET's. The BJT's might be considered lowly, just for beginners and rubes, but at least I get *something* from them. I'm going back to that simple 2N2222 circuit, put the 5th overtone tank on that collector, and just go from there.

Wow, that would be fantastic. I really appreciate it. If you like you can e-mail me at lanbergld@vcu.edu.

I'm too busy tonight to mess with the circuit much. But I have a question, if you don't mind: I just got a batch of small parts, including crystals. Can a 25 MHz crystal (I assume its fundamental) be used as an overtone crystal? Say at 5th overtone, 125 MHz? Or does the crystal have to be an overtone crystal? I read something recently that seemed to say a fundamental crystal can't be used at an overtone, but I'm not clear about that. Thank you.

Can a 25 MHz crystal (I assume its fundamental) be used as an overtone crystal? Say at 5th overtone, 125 MHz?

Maybe...

In theory, any crystal can be operated on one of its overtones. The overtones aren't exactly on the harmonics - they are mechanical vibrations. So if you order a 125 MHz overtonecrystal the fundamental won't be exactly on 25 MHz: they will grind it so the overtoneis on the right frequency.

Sometimes, however, a crystal can be damage or have limited activity (overtone oscillationrequires more feedback). It might be mounted in such a way that the vibrations aredampened, etc. So not every crystal will operate on an overtone (or operate easily),but many crystals will, even when designed for fundamental operation. As you get to the higher overtones (5th, 7th, etc.) it becomes more difficult to get a crystal to oscillateproperly unless it is specifically designed for such use

While you can find some specialty crystals with fundamentals as high as 50 MHz, generallycrystals over about 24 to 25 MHz will be overtone types. So your 25 MHz crystal mightbe a 3rd overtone - you'll have to try it and see.

Sounds like you have abandoned the idea of using the cable box for IF oscillator. FWIW, I found that the broadband cable modem that happened to be in my shack (it's where the cable comes in from the outside) generated wide-spectrum noise (plus one-frequency images) that virtually blanked out ham HF reception on several bands. In the end I had to lengthen the incoming cableco coax so that I could put the modem in another place a couple of rooms away.

Yes, I abandoned the cable box idea, and have commenced trying to build a VHF converter using a crystal oscillator, on a small plug in board. I'm getting somewhere. I can now get NOAA Weather radio (162.475 MHz) near 10-meters on my portable shortwave radio, but so far that's it. I think I need to improve the mixer portion.

I'm just making a basic rf input/l.o./mixer type of configuration. I don't have the experience or parts to get too fancy right now.

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